The present disclosure generally relates to carbon dioxide sequestration, and, more specifically, to methods for decreasing the quantity of free carbon dioxide that is present within a subterranean formation.
Carbon dioxide sequestration has recently been the subject of intense research and development activities in a number of fields, primarily due to its environmental impacts as a greenhouse gas. In this regard, significant pressure has been placed on various industries to decrease their carbon dioxide emissions in order to address the effects of global warming and ocean acidification. However, decreased carbon dioxide levels can be beneficial in a number of other applications.
A number of approaches have been implemented for capturing carbon dioxide from industrial processes before it reaches the atmosphere. These approaches can rely upon physical scrubbing of the carbon dioxide with a substrate or upon a chemical reaction that transforms the carbon dioxide into another form, so that it is more easily stored than a carbon dioxide gas. Chemical approaches can involve the reaction of carbon dioxide with ethanolic aqueous amine solutions or with metal cations. The captured carbon dioxide in the reaction product can subsequently be released, if desired, but the release process can be energy intensive and expensive. Physical scrubbing processes can involve the physical adsorption of carbon dioxide to the substrate. However, low adsorption capacities and limited selectivity for adsorption of carbon dioxide over other gases can hinder this approach. In many cases, adsorption selectivity to a substrate can decrease with increasing temperature, which can make this approach problematic for many industrial processes in which carbon dioxide is produced.
Carbon dioxide can originate in a subterranean formation from various sources. Downhole sources of carbon dioxide can include, for example, dissolution of carbonate minerals or carbonate scales with an acid. In some cases, carbon dioxide can be injected into a subterranean formation as a non-aqueous fracturing fluid or as a pressurizing fluid in enhanced oil recovery operations. Carbon dioxide can also be used to foam a treatment fluid that is introduced into a subterranean formation.
Although carbon dioxide can sometimes be intentionally introduced into a subterranean formation to produce one or more beneficial effects therein, there are certain instances in which the presence of carbon dioxide in a subterranean formation can be problematic. For example, when conducting an acidizing operation in a subterranean formation, the presence of excessive carbon dioxide can be especially problematic. One reason that the presence of excessive carbon dioxide can be problematic is due to its equilibrium with bicarbonate/carbonic acid in aqueous fluids. An exemplary reaction is shown in Formula 1 below that demonstrates the reaction of calcite (calcium carbonate) with hydrochloric acid in which this equilibrium is present.CaCO3+2HCl-->CaCl2+H2CO3<-->H2O+CO2  (Formula 1)The buffering effect produced by carbonic acid/bicarbonate can have significant implications during an acidizing operation in which a carbonate material is being dissolved. Namely, even if the acid completely spends in dissolving the carbonate material, the aqueous fluid can still have a pH of around 5 due to the buffering effect. Moreover, the presence of significant quantities of carbon dioxide can drive the equilibrium position of the reaction in the reverse direction, thereby slowing the dissolution of the carbonate material with the acid. In addition, the equilibrium induced by the presence of carbon dioxide can lower the effective concentration of the acid that brings about dissolution of the carbonate material.